Cr plus 2 yields Cr plus 3 oxidation has been used to fix Cr to Mg-binding sites in yeast enolase. Under some conditions substrate will then become tightly or covalently bound to the enzyme. We are investigating the physical properties and catalytic properties of such complexes to gain more understanding of the functions of the different metal sites on the enzyme. Attempts are being made to isolate peptides with substrate and metal bound. The role of pyruvate kinase in regulating human red cell metabolism is being studied. We have recently found that washed red cell membranes will catalyse phosphorylation and deactivation of the enzyme and have partially purified a protein phosphatase from some cells which dephosphorylates and reactivates pyruvate kinase.